The present invention relates to an illumination apparatus that efficiently collects radiation throughout a large solid angle from a source and redirects it through multiple components to maintain high brightness.
Many systems have been devised to collect and redirect radiation with high efficiency and brightness for a variety of purposes. A significant number of these systems have been devised for applications as diverse as hand-held flashlights and digital projection illumination systems. These generally fall into six different classes of approaches described below.
Simple Conical Reflectors: Simple conical reflectors are the oldest method available for the collection and redirection of light and have been addressed in textbooks for decades. They most often fall into one of three categories: spherical reflectors (κ=0) in which re-imaging a point results in an aberrated image of that point unless the point and its image both lie at the center of curvature; parabolic reflectors (κ=−1) in which only the point at the focus of the parabola is imaged back to an unaberrated point at infinity, and elliptical reflectors (−1<κ<0) in which only the point at the first focus of the ellipse is re-imaged at the second focus of the ellipse without aberration. In each case, large aberrations are encountered, and therefore performance is lost at all points other than the defining focus of the conic reflector. This is true even when these conics are used in combination with each other, or in combination with more conventional imaging refractors such as lenses.
Combinations of Conical Reflectors: Combinations of pure conical reflectors have also appeared in the literature in profusion, sometimes with aligned axes, sometimes with tilted axes. Thus, by way of illustration, reference may be had to U.S. Pat. No. 5,613,767, which teaches the use of combined spherical and ellipsoidal reflectors with collinear axes. This particular use of the spherical and ellipsoidal reflectors causes both to work under optimum conditions, but cannot compensate for the aberrations resulting from the physical (volumetric extent) of the source. This issue can be minimized by making the reflectors very large compared to the extent of the source, but this makes the system too bulky for many applications. Moreover, practical issues arise with regard to: thermal management of the lamp since it is essentially enclosed in a trapped air space; manufacturing costs of reflectors that can withstand the heat and have minimal expansion coefficients that would degrade performance; assembly costs associated with precisely aligning the two disparate reflector forms with the emission source; the specificity of the emission source since only a plasma lamp will allow the radiation re-imaged by the spherical component to pass through the emission region without detrimental absorption.
Reference also may be had, e.g., to U.S. Pat. No. 5,408,363, which circumvents some of these problems in its description of blended parabolic reflectors with non-coincident axes. The thermal concerns of this system are relatively manageable compared to the former system, and the manufacturing and assembly concerns are mitigated in the tooling for the reflector. There is furthermore no attempt in this system to re-image the source back onto itself, so the specificity restriction is avoided. However it is clearly stated that the attempt of the invention is to solve the radiation redirection problem solely with the purely conic reflector system. These systems will once again suffer the aberration-induced performance loss characteristic of all pure conic reflectors when used with radiation sources larger than a point.
Reference also may be had, e.g., to U.S. Pat. No. 5,136,491, which is similar to U.S. Pat. No. 5,408,363 in that it teaches the construction of a single reflector that blends two coaxial conic reflectors together along a line of intersection. These systems will once again suffer the aberration-induced performance loss characteristic of all pure conic reflectors when used with radiation sources larger than a point.
By way of further illustration, U.S. Pat. No. 6,318,885 describes a combination of discrete conic reflectors with non-coincident axes to enhance the performance of light collection with the intent of refocusing some of the emission of the source back into the source. One of the fundamental difficulties with this approach is the thermal load placed upon the lamp structure by increasing the radiation load on the surfaces. The increased thermal load often results in reduced lamp life. This system will once again suffer the aberration-induced performance loss characteristic of all conic reflectors when used with radiation sources larger than a point.
Conical Reflectors with Departures: Referring again to the alternative means of collecting light, conical reflectors with departures may be used. Departures from the basic conic reflector have also been described in the literature. Thus, e.g., U.S. Pat. No. 6,302,544 B1 describes a paraboloidal reflector with surface deformations specifically applied to adapt it to a lens array. It specifically defines a parabolic base reflector used in conjunction with a source emanating from a point. The surface of a parabola is deviated in such a way as to uniformly illuminate multiple optical elements rather than to improve the brightness of the system.
Faceted Reflectors: Another alternative light-collecting means is faceted reflectors, which have been described, for instance in U.S. Pat. No. 5,123,729, where the radiation from the source is captured by individual facets of the reflector and redirected to a plane where the flux from each facet is superimposed so as to create a uniformly illuminated rectangular patch with minimal light lost outside of the defined aperture.
Non-Imaging Optical Systems: Yet another alternative light-collecting means is non-imaging optical systems, which have been described especially to make use of extended sources such as fluorescent tubes. See, e.g., U.S. Pat. No. 4,915,479, which describes such an optical system intended to efficiently utilize radiation from high efficiency phosphor light sources. These devices have not been applied effectively to collect light from quasi-point source emitters.
Conical Reflectors: One may also utilize conical reflectors as a light collecting means in combination with lenses, which have been described for illumination purposes. See, e.g., U.S. Pat. No. 5,857,041, where illumination of a manifold of optical fibers through a manifold of lenses is described. In U.S. Pat. No. 5,833,341, the lens is used to nominally collimate the output of an ellipsoidal reflector. The zonal variance is addressed by using an annular flat reflector to reverse some of the rays through the lens, the glass envelope of the lamp, and the emitter. In so doing, it is hoped that they will strike a more favorable zone of the reflector. In theory, this may be perceived to be effective, but several problems are encountered in practice. The first of these is the additional thermal loading caused by the reversed energy impinging on envelope and electrodes. The second is that the angles of the rays reflected by the annular ring will not permit the energy to be re-imaged exactly into the gap of the electrodes. The bulk of this energy is re-imaged onto the electrodes causing overheating of the lamp, premature erosion of the electrodes, and often explosion of the lamp due to increased gas pressure. Such re-imaging of the arc should be avoided unless it can be proven to be done efficiently and reliably over the entire lifespan of the lamp. At the least, it is unfeasible for any source but an arc lamp with a thin plasma.
As is known to those skilled in the art, basic illumination systems are comprised of a source of emitted radiation, and a collection system. The metric defining the best design for a particular application is usually determined by several competing parameters, some practical, some fiscal, and some technical. The first two are most often addressed by required package dimensions, materials cost, manufacturing costs, and assembly and alignment costs.
The most important technical issue in designing illumination systems is to achieve high collection efficiency while holding the physical property of the optical Lagrange Invariant, better known as the etendue, of the system to a minimum. The etendue has been mathematically defined and justified in the literature as a characteristic of all optical systems. (See, for instance, Modern Optical Engineering, Warren J. Smith) In one of its more useful forms, the etendue ε of an illuminated panel is defined by the illuminated area and the solid angle through which the illumination arrives:ε=π·NA2·Awhere NA is the sine of the half angle of the illumination, and A is the area illuminated. This quantity will usually inflate as one propagates radiation through an illumination optical system due to poor design, resulting in reduced brightness. Designing an illumination system beginning with a source of low etendue is clearly advantageous.
A source with maximum power emitted from a minimal volume is desirable in order to begin with low etendue. For this reason, most critical illumination systems for visual use make use of a compact plasma arc lamp such as a high pressure mercury lamp.
FIG. 1 is a plot of basic geometry, structure, and radiation pattern of a typical compact plasma arc lamp presented in spherical coordinates. Referring to FIG. 1, the three dimensions are radial position in the plane of FIG. 1, angular position θ in the plane of FIG. 1, and angular position φ in a direction disposed perpendicularly to the plane of FIG. 1. It can be seen that while the luminance varies greatly as a function of the angle θ, it characteristically varies only slightly as a function of the angle φ. If the lamp axis is aligned with the optical axis, and the collecting aperture subtends 130–140 degrees in θ, nearly all of the light from the lamp is collected. Additionally, the distribution of luminance within the arc gap itself is of great importance.
FIG. 2 is a plot of a characteristic luminance distribution for an AC arc lamp presented in Cartesian coordinates. Since this distribution varies with lamp type, arc gap, power level, and whether or not a DC or an AC lamp is employed, the impact of emitter size on the design of the collection system must be considered.
In prior art light sources comprising a lamp and an elliptical reflector, such elliptical reflector forms an imperfect image of the lamp that is disposed along the axis thereof, and the degree of imperfection is in part dependent on the ratio of source extent to the base radius of the elliptical reflector. Such an imperfect image renders the light source unsatisfactory for many uses that require a source having a uniform light distribution therefrom.
It is therefore an object of this invention to provide a light collector for use with a lamp, which directs light from such lamp in manner that is highly collimated (i.e. narrow angle) and has a small cross-section.